Internal combustion engines produce soot during the combustion process. This is known for diesel engines and has led to the development of corresponding soot filters and exhaust gas aftertreatment systems for diesel engines. However, internal combustion engines with spark ignition, in particular gasoline engines with direct injection, produce soot particles during operation which may enter the ambient air with the exhaust gas. Even though the amount of soot particles produced in gasoline engines is generally lower than in diesel engines, it is desirable to prevent or at least reduce the release of soot particles and the pollution of the ambient air caused thereby.
Soot particles can be removed from the exhaust gas by a particle filter. Such a particle filter may be an additional component part having a corresponding spatial requirement and additional costs. It is therefore advantageous to integrate the function of the particle filter with the other components for exhaust gas aftertreatment.
It is known from EP 1 055 805 B1, in the case of a diesel engine, to separate the soot in a plurality of successive method stages at a filter element for soot, which filter element is provided with a catalyst coating for oxidation of the nitrogen monoxide contained in the exhaust gas into nitrogen dioxide. According to EP 2 273 079 A1, an exhaust gas aftertreatment device is equipped with an insert which is formed in a section on the inflow side as an oxidation catalyst and in a section on the outflow side as a particle filter. In WO 2008/107423 A1 a diesel particle filter with a filter body formed of a ceramic material is disclosed, which in a filter section comprises planar and porous filter walls for the exhaust gas to flow through and which, in addition to the filter section, comprises a catalytic section having an oxidizing catalyst coating or an NOx storage coating.
In gasoline engines it is known to integrate the function of a corresponding particle filter (gasoline particle filter, GPF) with that of a three-way catalyst (TWC) generally provided in the exhaust gas system in any case in gasoline engines. By applying the catalytically active material of the three-way catalyst to the filter walls of the exhaust gas ducts of a particle filter, however, the resistance posed by the particle filter to the throughflow of the exhaust gas, and thus the exhaust gas counter pressure, would be considerably increased. In contrast to a conventional three-way catalyst, the catalytically active material is therefore introduced into the porous filter walls of the particle filter, thus reducing the pressure loss and therefore the exhaust gas counter pressure.
However, the inventors herein have recognized a few issues with the above approaches. For example, it has been found that the insertion of the catalytic coating in the filter walls of the particle filter (GPF) leads to impaired starting behavior compared to conventional three-way catalysts (TWCs). In particular, a longer period is necessary until a sufficient catalytic effect for conversion of the exhaust gas pollutants is achieved, and therefore the emission of pollutants, in particular hydrocarbons, by motor vehicles provided with such a filter is increased on the whole. It would be possible to improve the starting behavior by increased heating of the exhaust gas aftertreatment device, for example by increasing exhaust gas enthalpy or by electric heating, however fuel consumption would be increased as a result. It would also be possible to improve starting behavior by way of an increased noble metal content of the catalyst material, but this would increase costs.
Thus, embodiments are provided to at least partly address the above issues. In one example, an exhaust gas aftertreatment device for a gasoline engine comprises a filter body with porous filter walls through which exhaust gas flows to remove soot, the porous filter walls containing a first catalyst material and having a coating of a second catalyst material on partial areas of the filter walls.
In this way, particulate matter may be removed from the exhaust via the porous filter walls. Further, the catalyst material contained in the filter walls as well as coating the walls may act to convert emissions in the exhaust. By combining the particulate filter with the catalyst material, a single exhaust aftertreatment device may be provided rather than a separate catalyst and particulate filter, reducing engine packaging space. By coating only partial areas of the filter walls, rapid catalyst light-off may be achieved without increasing exhaust back-pressure excessively, thus avoiding fuel economy penalties.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.